Luminous quartz resonators, invented by Giebe and Scheibe, have
been in use in Germany for several years. These resonators are
visual indicators of frequency and may be used to check the performance
of calibrated circuits, or of a transmitter; they depend for
their operation on the well-known piezo-electric properties of
quartz crystals. A crystal is lightly held in a metal fitting
which constitutes one of the electrodes in a gas discharge tube,
the other electrode being a wire or small metal plate near the
surface of the crystal. The usual gas filling is neon at about
3 to 5 mm. pressure. The metal fitting, holding the quartz, is
constructed so that no part of it is as close to the other electrode
as the surface of the quartz.
With the electrodes connected appropriately
to a resonant circuit, powerful oscillations will cause the neon
to glow brightly with the usual insensitive glow discharge between
the electrodes. If, however, the input is considerably reduced
in strength, so that direct glow discharge between the electrodes
cannot be produced, then a much fainter but very distinct glow
appears on the surface of the quartz when its resonant frequency
is applied. This characteristic glow is due to the field produced
by the piezo-electric charge on the surface of the quartz, and
the sensitivity of this visual indication of oscillations is
It has been found convenient to
mount the resonators in ordinary 4-pin valve bases, the electrodes
carrying the quartz being connected, one to the grid pin and
the other to the anode pin. The figure above shows three samples.
The short one, with a horizontally supported bar of quartz, is
similar in general appearance to the original German models and
is typical of resonators with frequencies of about 100 kc/s.
The largest type was constructed for lower frequencies of 40-50
kc/s. The one with the square crystal slice is designed to work
on a frequency of about 3,000 kc/s.